Creating a virtual reality experience isn’t like making other video games because you have to consider so many other details … like the sound R2-D2 makes when you slap him.

Industrial Light & Magic, the Hollywood special effects house responsible for the visuals in Star Wars and many other blockbusters, gave a presentation at the Game Developers Conference this week. It talked about the importance of making simulated environments interactive. ILMxLab, the company’s VR and AR division, has a new Star Wars “cinematic experiment” for HTC’s Vive head-mounted display. While making that game, the company discovered that it people loved banging on an old ship component or R2-D2. The demo would respond with a satisfying sound effect and rumbling from the controller.

ILMxLab boss Rob Bredow said that his team would see people reach out and try to hit things like boxes, the Millennium Falcon, and R2-D2 during their tests. Nothing would happen. Bredow and his team realized that they should deliver something in those situations for the player.

“With a little bit of haptics and a simple sound effect, you give people a deep level of immersion,” he said. “When users discover this, they come away satisfied.”

Bedrow then showed a clip of someone using the HTC Vive’s motion controllers to slap R2-D2. His dome gave off a resounding thud, and it really looked like the controller was interacting with the digital character.

It’s these types of tricks that’ll convince people that VR is something different and special.